Br?nsted Acid-Catalyzed Carbocyclization of 2-Alkynyl Biaryls

Article abstract of DOI:10.1002/adsc.201801526

Ortho-alkynyl biaryls react in the presence of catalytic amount of Br?nsted acids to give phenanthrenes in high yields under mild conditions. The activity and selectivity of this transformation are governed by the substitution pattern of the diarylalkyne moiety. Selectivity shifts are observed between the carbophilic Lewis and Br?nsted acid-catalyzed cycloisomerization involving alkyne activation. (Figure presented.).

Full text of DOI:10.1002/adsc.201801526

Title: Brønsted Acid-Catalyzed Carbocyclization of 2-Alkynyl Biaryls
Authors: Julien Gicquiaud, Antoine Hacıhasanoğlu, Philippe
Hermange, Jean-Marc Sotiropoulos, and Patrick Toullec
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10.1002/adsc.201801526
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201801526
Brønsted Acid-Catalyzed Carbocyclization of 2-Alkynyl Biaryls
Julien Gicquiaud,a Antoine Hacıhasanoğlu,a Philippe Hermange,a Jean-Marc
Sotiropoulos,b Patrick Y. Toullec*a
a
University of Bordeaux, ISM (CNRS−UMR 5255), 351 cours de la Libꢀration, 33405 Talence Cedex, France
E-mail address: patrick.toullec@u-bordeaux.fr
CNRS/UNIV PAU & PAYS ADOUR, Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement
b
et les Matꢀriaux (IPREM, UMR 5254), Hꢀlioparc, 2 avenue du Prꢀsident Angot, 64053 Pau Cedex 09, France
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. Ortho-alkynyl biaryls react in the presence of
catalytic amount of Brønsted acids to give phenanthrenes in
high yields under mild conditions. The activity and
selectivity of this transformation are governed by the
substitution pattern of the diarylalkyne moiety. Selectivity
shifts are observed between the carbophilic Lewis and
Brønsted acid-catalyzed cycloisomerization involving
alkyne activation.
Regioselectivity of the cycloisomerization
represents a major issue as dibenzofulvenes,
resulting from a 5-exo hydroarylation, were often
observed as side products.
Keywords: Alkynes; Brønsted acid catalysis;
Phenanthrenes; Electrophilic activation;
Cycloisomerization
The
6-endo
cycloisomerization
of
2-
alkynylbiphenyl represents one of the most
straightforward methodology to access the
phenanthrene core via an atom-economical route.
Such intramolecular hydroarylation reactions
have been thoroughly investigated via a large set
of reactions conditions, reagents or catalysts.[1]
The transformation of the parent substrate
proceeds by thermal activation (700°C), to
deliver quantitatively a mixture of phenanthrene
and benzazulene (scheme 1, eq. 1).[2] This
reaction has also been achieved via an oxidative
radical cyclisation[3] in the presence of the
Bu3SnH/AIBN couple of reagents in toluene at
reflux, followed by a protodestannylation in the
presence of HCl (scheme 1, eq. 2). Metal-
Scheme
cycloisomerisation reactions.
1.
Strategies
for
ortho-alkynylbiaryls
catalyzed
cycloisomerizations
of
2-
alkynylbiaryls were extensively studied since the
seminal work of the group of Fürstner.[4] Two
competitive pathways involving either (i) a metal
vinylidene intermediate[5] or (ii) carbophilic π-
activation of the alkyne[6] have been proposed to
explain the reactivity observed with the different
transition metals (scheme 1, eq. 3).
To date, significant catalytic activities have been
obtained in the presence of metal salts or
complexes obtained from the following
elements: Pt,[4,7] Au,[4,8] In,[4] Ga,[4] W,[5b] Ru,[5a,9]
Pd,[10] Nd,[11] Cu,[12] Ag,[13] Fe,[14] and Al.[15] In
most studies, metal catalysts only operate at high
1
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10.1002/adsc.201801526
Advanced Synthesis & Catalysis
temperatures (from 80 to 120°C) with catalyst CHCl3, toluene, cyclohexane and MeCN (table 1,
loadings ranging from
Stoichiometric amounts
5
to 20 mol%. entries 8-11). In contrast, total inhibition of the
electrophilic acid catalysis was observed in diethyl ether
of
iodination reagents or excess Brønsted acids (table 1, entry 11). Using the procedure of
have also been reported as efficient mediators for Hintermann[22c] for the in situ formation of triflic
this cycloisomerization of 2-alkynylbiaryls, acid from AgOTf and tert-BuCl in a 1:4 ratio, a
following the early investigation of the group of good conversion was also observed (table 1,
Swager[16] on the use of main group electrophiles entry 13 vs 5).
in these transformations[17] (scheme 1, eq. 4).
Table 1. Optimization of reaction conditionsa
Over the years, this methodology showed its
versatility and allowed the synthesis of a variety
of polycyclic aromatic hydrocarbon skeletons.[18]
The phenanthrene moiety is also present in a
large array of biologically active molecules[19]
and in materials possessing electronic and optic
properties.[16,20] Nevertheless, the use of a
catalytic amount of Brønsted acid in this
transformation was overlooked.[21] Considering
the working hypothesis that some of the metal-
catalyzed reactions may indeed proceed under
hidden Brønsted acid catalysis,[22] we decided to
engage in the study of the cycloisomerization of
2-alkynylbiaryls in the presence of Brønsted acid
catalysts in order to assess the synthetic utility of
this metal-free strategy.
Ent Cat.
ry
Cat.
loading
Solvent Time Conv. Yield
(h)
b (%)
c (%)
1
TfOH
5
CH2Cl2
2
100
100
73
0
100
19
99
99
n. i.
-
89
n. i.
n. i.
99
99
80
99
-
2
3
Tf2NH
5
5
5
0.1
0.01
0.1
5
5
5
CH2Cl2 70
CH2Cl2 70
CH2Cl2 70
CH2Cl2 15
CH2Cl2 16
CH2Cl2 15
3d
4
HCl
5e
6e
7
TfOH
TfOH
Tf2NH
TfOH
TfOH
TfOH
TfOH
TfOH
27
8
9
10
11
12
13
CHCl3
toluene 24
C6H12 24
CH3CN 24
24
100
100
100
100
100
80
As a model experiment, the cyclization of 1a was
engaged at room temperature in the presence of
various Brønsted acids (Table 1). Using 5 mol%
of trifluoromethanesulfonic acid[23] (TfOH) in
dichloromethane, the obtention of phenanthrene
2a in a quantitative yield was observed after 2h
at room temperature. Full conversion was also
observed with 5 mol% of bis(trifluoromethane-
sulfonyl)amine (Tf2NH) whereas 73% of
5
5
Et2O
CH2Cl2
24
4
AgOTf/ 1/4
n. i.
tBuCl
a) Reaction run on 0.1 mmol of 1a in 2 mL of solvent.
b)
1
c)
Determined by H NMR analysis of the crude mixture.
Isolated yields after column chromatography on silica gel.
d) 3: 4-nitrobenzenesulfonic acid. e) Reaction run using 5
mmol of 1a in 20 mL of solvent. n. i.: not isolated.
conversion
was
obtained
with
4-
nitrobenzenesulfonic acid 3 after 70h of reaction
(table 1, entries 2-3). Finally, no reaction was
observed employing HCl as a catalyst (table 1,
entry 4). Thus, TfOH was selected and the
influence of the catalyst loading was further
investigated. Full conversion and an isolated
yield of 89% of 2a was obtained using 0.1 mol%
Having these optimized conditions in hands, we
decided to investigate the scope and limitations
of this transformation (scheme 2). The reaction
accommodated a variety of electron-donating
groups on the distal aromatic ring (2c-h), but
also worked with a phenyl substituent 2b. The
reaction proceeded with a modest 44% yield in
the case of the 4-dimethylaminophenyl
substituent under forcing conditions (2i), but
failed with the free amino group (1n). The
reaction also proceeded for substrate 1j bearing a
fluorine atom on the para position of the
terminal phenyl ring. In contrast to the results
obtained with aromatic rings bearing electron-
donating substituents, no reactivity could be
1
of this catalyst after 70 h, whereas 19% of H
NMR yield was observed using 0.01 mol% of
TfOH after the same time, corresponding to a
turnover number of 1900 (table 1, entries 5-6).
Surprisingly, at low catalysts loadings, Tf2NH,
the strongest Brønsted acid, turned out to exhibit
a lower activity (table 1, entry 7). Solvent effect
was studied fixing the reaction time to 24 h: the
reaction took place with full conversion in
2
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10.1002/adsc.201801526
Advanced Synthesis & Catalysis
monitored with the para-nitro substrate (1o). substrate (see substrate 1r for example). In
Regarding the nucleophilic aromatic ring, contrast, a substrate possessing an internal alkyl
electron-rich anisole derivatives gave excellent substituent cleanly cyclized to deliver the
yields (2k-l) whereas electron-poor benzonitrile- corresponding 2H-chromene 2s in good yield.
containing substrate 2m required harsher
reaction conditions (80°C). Terminal alkynes and
silyl-substituted substrates 1p and 1q exhibited
no reactivity under these mild conditions.
The chemoselectivity observed for these
transformations was excellent and favors
exclusively the aromatic carbon nucleophile over
O-nucleophiles. For example, the formation of
phenanthrene 2f from substrate 1f occurred
selectively by a 6-endo cyclisation involving the
aromatic ring (scheme 3, eq. 1). As the group of
Alami[24] previously showed that tolane
substrates bearing a 2-methoxy substituent could
cleanly cyclize to give benzofuran products in
good yields in the presence of a stoichiometric
amount of para-toluenesulfonic acid (PTSA) in
ethanol at 130°C under microwave irradiation,
we treated substrate 1f under these conditions
and observed the formation of a mixture of the
phenanthrene 2f and benzofuran 5 resulting from
the C- and O-nucleophile attack on the alkyne in
a 77:23 ratio with a moderate conversion
(scheme 3, eq. 2). More interestingly, when the
free ortho-phenol substrate 1t was reacted in
presence of 5 mol% of TfOH, this alkyne was
also cleanly transformed to the phenanthrene 2t
in 94% yield (scheme 3, eq. 3). This result under
Brønsted acid catalysis was in sharp contrast
when compared to the previously described
metal-catalysed cyclisations.[25] Indeed, when
treated with 20 mol% PtCl2 at 80°C in toluene
for 19 h, 1t exclusively furnished the
corresponding benzofuran 5 (99% yield, scheme
3, eq. 4). To continue the comparison with the
carbophilic
Lewis
acid
cyclisation
of
alkynylbiphenyls, we prepared the substrate 1u
used by Fürstner in his study of the transition
metal-catalyzed synthesis of phenanthrenes.[4a] In
the presence of 5 mol% PtCl2 in toluene at 100°C
for 24h, this derivative was reported to cyclize to
deliver a mixture of the phenanthrene 2u and the
dibenzofulvene 6 in 87% yield and 60/40
regioisomeric ratio, resulting from a 6-endo and
5-exo cyclization respectively (scheme 3, eq. 5).
Remarkably, in the presence of catalytic TfOH,
the product was obtained in 91% yield as an
unseparable mixture of 2u and 6 in a 95/5 ratio.
a)
Reaction conditions: 0.1 mmol of substrate and 5 mol%
of TfOH in CH2Cl2 (0.05 M). Isolated yield after column
b)
c)
chromatography on silica gel. 20 mol% of TfOH used.
Reaction run at 80°C in (CH2Cl)2 (0.05 M). r.r. regiomeric
ratio.
Scheme 2. Scope of the Brønsted acid-catalyzed
phenanthrene synthesisa
The catalytic conditions were also not
compatible with the presence of an alkyl
substituent on the terminal position of the
3
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10.1002/adsc.201801526
Advanced Synthesis & Catalysis
Figure 1. Reaction profile of the cyclization of 1a to 2a
catalyzed by TfOH and PtCl2.
Such observations go against the accepted view
that carbophilic Lewis acids require softer
reactions conditions and exhibit higher
selectivities than Brønsted ones. In the case of
1,2-diarylalkynes substrates with carbophilic
acids such as Pt, the formation of a π complex A
between the metal fragment and the carbon
unsaturation is observed (scheme 4). The
polarisability of these Lewis acids induces a
slippage of the metal fragment occurring
reversibly on both ends of the C-C triple bond.
The consecutive nucleophilic attack might
therefore take places on both sites leading to a
mixture of vinyl metal isomers B and C. Finally,
protodemetallation completes the catalytic cycle
and gives D and E.[6] This situation implies that
depending on the stability of the final products,
the use of these catalysts may lead to the
selective formation of the anti Markovnikov
product, as observed with 1r and PtCl2 (Scheme
3, eq. 4). On the other hand, the substitution
pattern of the aryl groups, i.e. the presence of
electron-donating groups, polarizes the C-C
triple bond and thus induces its selective reaction
with a proton delivering the vinyl carbocation
F.[26,27] Trapping of this cationic intermediate
with a nucleophile would deliver product D, i.e
resulting from a 6-endo cyclization for 2-
alkynylbiaryls substrates. For example, DFT
calculations of vinyl cations F and F’ revealed
that intermediate F is more stable by 4.86
kcal/mol in the case of 1s (see supporting
information for details). These results explain
that the 6-endo cyclization, corresponding to a
Markovnikov addition, is strongly favored
whereas the 5-exo cyclization hardly takes place
in the Brønsted acid-catalyzed intramolecular
arylation of diarylalkynes under these mild
reaction conditions.
Scheme 3. Chemo- and regioselectivity of the Brønsted
acid catalysed cyclization.
To further compare the activity of PtCl2 and
TfOH catalysts, the kinetics of the
transformation from 1a to 2a (0.05 M) was
recorded (Figure 1). With TfOH, the initial
reaction rate was found to be of 1.23 10-3 mol.L-
1.min-1 at room temperature, whereas in the
presence of PtCl2 at 80°C, the reaction rate was
only of 1.2 10-5 mol.L-1.min-1. Thus, with a
difference of two orders of magnitude without
taking in account the temperature effect, the
TfOH catalytic system revealed to be much more
active than the PtCl2 one.
4
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10.1002/adsc.201801526
Advanced Synthesis & Catalysis
This work was supported by the Centre National de la Recherche
Scientifique (CNRS) and the Ministère de l’Enseignement
Supérieur et de la Recherche. J. G. acknowledges the Ministère
de l’Enseignement Supérieur et de la Recherche for a PhD grant.
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a solution of trifluoromethanesulfonic acid in CH2Cl2 (C =
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give product 2.
CCDC-1828671
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Advanced Synthesis & Catalysis
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6
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Advanced Synthesis & Catalysis
COMMUNICATION
Brønsted Acid-Catalyzed Carbocyclization of 2-
Alkynyl Biaryls
Adv. Synth. Catal. Year, Volume, Page – Page
Julien Gicquiaud, Antoine Hacıhasanoğlu, Philippe
Hermange, Jean-Marc Sotiropoulos, Patrick Y.
Toullec*
7
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